1. Field of the Invention
This invention relates to a pickup apparatus for detecting (reading out) signals recorded on a magneto-optical recording medium such as a magneto-optical disk. This invention particularly relates to a pickup apparatus using an optical waveguide.
2. Description of the Prior Art
In recent years, magneto-optical recording media such as magneto-optical disks are widely used as recording media for image signals, audio signals or the like. Signals recorded on the magneto-optical recording medium in terms of directions of magnetization are detected by use of an optical pickup apparatus. The optical pickup apparatus is constituted to irradiate, for example, linearly polarized light such as a laser beam to the surface of the magneto-optical recording medium, and to detect the directions of magnetization on the magneto-optical recording medium by utilization of the magnetic Kerr effects with which the plane of polarization of the light reflected by the magneto-optical recording medium is rotated in accordance with the direction of magnetization.
Specifically, with the pickup apparatus for the magneto-optical recording medium, the light reflected by the magneto-optical recording medium is detected by a photodetector via an analyzer, and the directions of magnetization, i.e. the recorded signals, are detected by utilizing such effects that the detected optical amounts change in accordance with the rotation of the plane of polarization of the reflected light. Besides the function of carrying out detection of the recorded signals as mentioned above, the pickup apparatus is required to have the function of detecting tracking error, i.e. the function of detecting whether the light beam for the detection of the condition of magnetization is deviated rightward or leftward from the center of a track along a predetermined groove, and the function of detecting focusing error, i.e. the function of detecting whether the focusing point of the light beam is deviated forward or backward from the reflection surface of the magneto-optical recording medium. A signal representing the tracking error or the focusing error is utilized to control tracking or focusing so that the signal is canceled, thereby to irradiate the light beam accurately to the predetermined track or to focus the light beam accurately onto the reflection surface of the magneto-optical recording medium. As the method of detecting a tracking error, there have heretofore been known the push-pull technique, the heterodyning technique, temporal (time difference) detection technique and other such techniques. On the other hand, as the method of detecting a focusing error, there have heretofore been known the astigmatism technique, the critical angle detection technique, the Foucault technique and other such techniques.
In order to provide the aforesaid functions together with the signal detecting function, the conventional pickup apparatus for a magneto-optical recording medium is composed of small optical elements such as a beam splitter for splitting a light beam reflected by a magneto-optical recording medium from the light beam irradiated onto the magneto-optical recording medium, a lens for converging the reflected light beam in the vicinity of a photodetector such as a photodiode, the aforesaid analyzer, and a prism for carrying out the tracking error detecting method and the focusing error detecting method.
However, the aforesaid small optical elements require accurate machining or processing, and troublesome adjustment of positions of the small optical elements with respect to each other must be carried out at the time of assembling the pickup apparatus. Therefore, the pickup apparatus using such optical elements naturally becomes expensive. Also, the pickup apparatus having the configuration as mentioned above is large and heavy, and therefore is disadvantageous from the viewpoint of making the read-out apparatus small and light and shortening the access time. Particularly, in the case where differential detection for improvement of the S/N ratio of the read-out signals is carried out, a semi-transparent mirror or the like is necessary for splitting the reflected light beam into two beams, and some optical systems for the differential detection require two analyzers. Therefore, it this case, the pickup apparatus becomes more complicated, larger and heavier.
Accordingly, various attempts have heretofore been made to eliminate the aforesaid drawbacks of the conventional pickup apparatus by simplifying the configuration of the pickup apparatus by use of, for example, a special optical element such as an aspherical lens. However, the optical element of this type is very expensive, and therefore the cost of the pickup apparatus using such an optical element does not become much lower than the cost of the aforesaid pickup apparatus even though the configuration is simplified.